At present, according to the universalization of mobile communication and activation of wireless broadband data communication, various frequency bands are used as available frequency bands in order to sufficiently ensure frequency band which is insufficient. The mainly used frequency bands are a low frequency band (698-960 MHz) and a high frequency band (1.71-2.17 GHz or 2.3-2.7 GHz). In addition, a multiple input multiple output (MIMO) technology based on a multiband antenna is an essential technology for increasing data transmission speed and is being applied to recent mobile communication network systems such as long term evolution (LTE) and Mobile WiMAX.
However, to install a plurality of antennas in order to support MIMO in the various frequency bands causes limitations in terms of tower space in which an antenna is installed in real outside environment as well as an increase in installation costs. Thus, a multiband antenna such as a dual band antenna or a triple band antenna is necessarily required. The multiband antenna has a structure in which a high frequency band antenna is inserted in the same space as that used for installing a low frequency band antenna, while maximally reducing an interference effect between elements, so as to maximally efficiently design an antenna area, especially, the width of the antenna. An example of such a multiband antenna is the earlier application by the present applicant in Korean Patent Publication No. 10-2010-0033888 (Title: “Dual band dual polarized antenna for a mobile communication base station”, inventors: Youngchan MOON, Ohseok CHOI, Published: described in the Mar. 31, 2010).
Generally, a multiband antenna, as described in Patent Publication NO. 10-2010-0033888, has a structure in which first radiation modules of a low frequency band and second and/or third radiation modules of a high frequency band are properly placed on at least one reflector erected in the lengthwise direction. For example, the first radiation modules may be vertically arranged in a row, and the second and/or third radiation modules may be vertically arranged on the left and right sides of the first radiation elements in a row, respectively. At this time, generally, each of the first radiation modules, the second radiation modules, and third radiation modules is combined in four directions of four radiation elements and, overall, is arranged with an angle of +45 and −45 degrees with respect to verticality (or horizontality), thereby generating two linearly polarized wave which are orthogonal.
Meanwhile, recently, as a radiation element and radiation module having a broadband characteristic have been required, a radiation element including a band where about 45 percent of the band is a fractional band width has been provided. The radiation element, for example, may have an operation characteristic of 1710-2690 MHz bands. In case of implementing the multiband antenna using a broadband radiation element, an interference problem between elements of each band is seriously on the rise, thus, this problem causes difficulty which is insurmountable at the time of efficiently designing a multiband antenna.